Chapter 16.

Chemical Reactions

Rates and Equilibria

The rate of a chemical reaction shows how fast it goes.

The equilibrium position of a chemical reaction shows how far it goes.

How Do Chemical Reactions Occur?

Collision theory is a set of statements that explains how chemical reactions occur. It is an extension of the kinetic molecular theory.

Kinetic molecular theory states that atoms, molecules, and ions, are in constant ran-dom motion. They collide with each other. Their average kinetic energy increases with temperature.

Collision Theory

Reacting particles must collide with each other in order to react.

Colliding particles must possess a minimum energy, called the activation energy, in order to react.

Colliding particles must collide with the proper orientation in order to react.

Collision Theory

NO2 + CO NO + CO2

Collision Theory

NO2 + CO NO + CO2

Collision Theory

Why do Chemical Reactions Occur?

Chemical reactions that are spontaneous occur to lower the energy of the system.

They do this by maximizing favorable inter-actions between particles of opposite charge, while minimizing unfavorable interactions between particles of like charge.

Reaction Energy

An endothermic reaction requires continuous energy input to continue.

6 CO2 + 6 H2O + light C6H12O6 + 6 O2

H = 2816 kJ

An exothermic reaction releases energy as it occurs.

C6H12O6 + 6 O2 6 CO2 + 6 H2O + heat

H = 2816 kJ

Reaction Energy

Reaction Energy

The strength of the chemical bonds that form or are broken in a reaction determines whether it will be exothermic or endothermic.

Breaking strong bonds absorbs more energy than breaking weak bonds.

Forming strong bonds releases more energy than forming weak bonds.

Reaction Energy

CH3OH + I1- CH3I + OH1-

Break CO 350 kJ/molForm CI 240 kJ/mol

110 kJ/mol

For every mole of CH3OH that reacts, 110 kJ must be supplied to the reaction.

Reaction Energy

CH3I + OH1- CH3OH + I1-

Break CI 240 kJ/molForm CO 350 kJ/mol

110 kJ/mol

For every mole of CH3I that reacts, 110 kJ will be released.

Reaction Energy

Reaction Rates

The rate of a reaction is the rate at which reactants are consumed or products are produced in a given time period.

Depends on:

Physical Nature of Reactants

Reactant Concentration

Reaction Temperature

Presence of Catalysts

Reaction Rates

Physical Nature of Reactants

Reaction Rates

Reactant Concentration

More particles mean more collisions!

Reaction RatesReaction Temperature

Reaction RatesCatalysts

Chemical EquilibriumWe've been writing chemical reactions as if

they go to completion, or until at least one reactant is consumed.

CH3OH + I1- CH3I + OH1-

But that only happens if the reaction releases a lot of energy. Usually things only go part of the way.

CH3OH + I1- CH3I + OH1-

Chemical Equilibrium

In most chemical reactions, a mixture of products and reactants form, and at some point their concentrations stop changing.

CH3OH + I1- CH3I + OH1-

There will still be a lot of CH3OH in the reaction mixture at that point.

Chemical Equilibrium

Chemical equilibrium is the state reached when concentrations of reactants and products remain constant over time.

Chemical equilibrium is reached when the rate of the forward reaction and the rate of the reverse reaction are equal.

Consider the reaction

H2(g) + I2(g) 2 HI(g)

Chemical Equilibrium

Chemical Equilibrium

Equilibrium Stoichiometry

0.0930 mol NO and 0.0652 mol Br2 are placed in a vessel. At equilibrium 0.0612 mol of NOBr is found. The reaction equation is:

2 NO(g) + Br2(g) 2 NOBr(g)

Init. 0.0930 mol 0.0652 0 mol

?? ?? ??

Final ?? ?? 0.0612 mol

Equilibrium Stoichiometry

0.0930 mol NO and 0.0652 mol Br2 are placed in a vessel. At equilibrium 0.0612 mol of NOBr is found. The reaction equation is:

2 NO(g) + Br2(g) 2 NOBr(g)

Init. 0.0930 mol 0.0652 0 mol

-0.0612 mol -0.0306 mol +0.0612 mol

Final ?? ?? 0.0612 mol

Equilibrium Stoichiometry

0.0930 mol NO and 0.0652 mol Br2 are placed in a vessel. At equilibrium 0.0612 mol of NOBr is found. The reaction equation is:

2 NO(g) + Br2(g) 2 NOBr(g)

Init. 0.0930 mol 0.0652 0 mol

-0.0612 mol -0.0306 mol +0.0612 mol

Final 0.0318 mol 0.0346 mol 0.0612 mol

Equilibrium Constants

The equilibrium constant is a numerical value that expresses the relationship between concentrations of reactants and concentrations of products for a system at chemical equilibrium.

Equilibrium Constants

The equilibrium constant is calculated from the reaction equation and the equilibrium concentrations of reactants and products.

2 NO(g) + Br2(g) 2 NOBr(g)

Keq = [NOBr]2 = 0.06122 = 107 [NO]2 x [Br2] 0.03182 x 0.0346

Equilibrium Constants

The equilibrium expression is the symbolic form for the concentrations of reactants and products that is evaluated to obtain the equilibrium constant.

a A + b B c C + d D

Keq = [C]c x [D]d [A]a x [B]b

Concentrations of solids and pure liquids are 1 M.

Equilibrium ConstantsExample:

1.00 mole of acetic acid (CH3COOH) is dis-solved to make 1.00 L of aqueous solution. It ionizes according to the equation:

CH3COOH + H2O CH3COO1- + H3O1+

At equilibrium,[CH3COO1-] = [H3O1+] = 4.2 x 10-3 M

What is Keq?

Equilibrium Constants

Example:

Calcium carbonate is the business end of hard water. The maximum concentration of Ca2+ and CO3

2- ions in hard water is 7.07 x 10-5 M. What is Keq for dissolving CaCO3 in water, to form a saturated solution?

CaCO3(s) Ca2+(aq) + CO32-(aq)

Equilibrium Constants

Example:

Lead(II) chloride is not very soluble in water. A saturated solution of PbCl2 is 0.0144 M in Pb2+ and 0.0288 M in Cl1-. What is Keq for dissolving PbCl2?

PbCl2(s) Pb2+(aq) + 2 Cl1-(aq)

Equilibrium Constants

So what do these things really tell me?

If Keq is large (>1000),

the concentrations of products are high relative to concentrations of reactants

the position of equilibrium lies to the right

the reaction is probably very exothermic

Equilibrium Constants

If Keq is small (<0.001),

the concentrations of reactants are high relative to concentrations of products

the position of equilibrium lies to the left

the reaction is probably very endothermic

Equilibrium Constants

Equilibrium Constants

If Keq is between 0.001 and 1000,

significant concentrations of both products and reactants are present at equilibrium.

H isn't large

Disturbing an Equilibrium

One can shift the position of an equilibrium by applying a stress to the system. A stress to an equilibrium is a change in conditions, such as addition or removal of one of the components of the equilibrium, or changing its temperature. For gases, changing a concentration will involve changing a pressure.

Disturbing an Equilibrium

LeChâtelier's Principle describes the behavior of an equilibrium which has been stressed.

If a stress (change in conditions) is applied to a chemical system at equilibrium, the system will readjust (change the position of the equil-ibrium) in the direction that best reduces the applied stress.

Disturbing an Equilibrium

3 H2(g) + N2(g) 2 NH3(g) + heat

What happens to the position of equilibrium if

[N2] is increased?

[H2] is decreased?

The pressure is increased?

[NH3] is decreased?

The system is heated?

The system is cooled?

Disturbing an Equilibrium

Being able to force a reaction that reaches equilibrium to completion is important. Separating the product(s) from reactant(s) takes time (time $!) and disposing of waste reactants is expensive.

C7H6O3 + CH3COOH C9H8O4 + H2O

Salicylic acid + acetic acid aspirin + water

Keq < 1, how do we get a high yield of aspirin?

Disturbing an EquilibriumWhat about catalysts?

Catalysts effect rate, not equilibrium.

Disturbing an EquilibriumWhat about catalysts?